Along with the rapid popularization of the internet in late years, there is a demand for further increase in the capacity of a communication traffic. Examples of the method of increasing the capacity of the communication traffic include the method of multiplexing the wavelength of signal light and increasing the number of channels transmitting an optical fiber at the same time (wavelength division multiplexing (WDM)) and the method of increasing the transmission quantity (transmission speed) for each channel per unit time. An example of the method of increasing the transmission speed includes the method of modulating the signal light at high speed.
Examples of the method of modulating the signal light at high speed include the method of directly modulating the signal light from a semiconductor laser, which is used as a light source and the method of indirectly (externally) modulating the signal light from the semiconductor laser. In the method of directly modulating the signal light, output light (signal light) from the semiconductor laser is directly modulated by modulating the drive current to the semiconductor laser. The method of directly modulating the signal light is used widely because the device configuration can be simplified. However, the modulation speed of the signal light by the method of directly modulating the signal light is limited by the response speed of the semiconductor laser. Therefore, the method of directly modulating the signal light is not suitable for modulating the signal light at higher speed. In contrast, in the method of indirectly (externally) modulating the signal light, in the condition where the semiconductor laser is operated by the direct current and the signal light intensity is kept constant, the signal light is modulated by an external modulator, which is another optical device. An example of the external modulator includes a Mach-Zehnder optical modulator (LN modulator) using a LiNbO3 crystal as a material. Optical transmission systems such as 10 Gb/s and 40 Gb/s using this LN modulator are in practical use. Further, since the LN modulator has a modulation characteristic independent from a wavelength, it is suitable for the WDM system.
However, the LN modulator has problems that the driving voltage thereof is high (for example, 5V or more) and the size thereof is large (for example, 10 cm or more). Hence, semiconductor optical modulators capable of reducing the voltage and size have been developed. Such semiconductor optical modulators use multiple quantum wells as core layers and utilize the refractive index change by voltage application by the quantum confined Stark effect (QCSE), for example. The semiconductor optical modulators use, for example, indium phosphide (InP)-based materials and gallium arsenide (GaAs)-based materials as materials for semiconductors. The Mach-Zehnder optical modulator (semiconductor MZM) using such a semiconductor optical modulator achieves the reduction in driving voltage (for example, 2 V) and the reduction in size (for example, about 2 mm), and therefore it has been in practical use in recent years. Further, the semiconductor optical modulator can be integrated, for example, with a semiconductor laser, other optical devices such as an optical amplifier, an electronic circuit, and the like.
As the semiconductor optical modulator, for example, a semiconductor optical modulator having a p-i-n-type layer structure as shown in FIG. 12 is used commonly (for example, see Patent Document 1). In a semiconductor optical modulator 120, formed is a Highmesa (deep ridge) waveguide structure in which an n-type semiconductor layer 122, an optical modulation layer 123, and a p-type semiconductor layer 124 are laminated on a substrate 121 in this order. A passivation (which works as insulation protection) film 129 is arranged at the side surfaces of the Highmesa waveguide structure. The arrow B indicates the boundary between the Highmesa waveguide structure and the passivation film 129. FIG. 14A shows an example of the band diagram of the semiconductor optical modulator 120. In a band diagram 120a, band diagrams 124a, 123a, and 122a are band diagrams respectively corresponding to the p-type semiconductor layer 124, the optical modulation layer 123, and the n-type semiconductor layer 122 of the semiconductor optical modulator 120 shown in FIG. 12. In the semiconductor optical modulator 120, the optical modulation layer 123 is interposed between the n-type semiconductor layer 122 and the p-type semiconductor layer 124. Signal light is guided into the optical modulation layer 123, and the signal light guided undergoes a refractive index modulation by a voltage applied to the optical modulation layer 123. The optical modulation layer 123 uses a multiple quantum well as a core layer and generally is an undoped layer (i layer). Since the undoped layer (i layer) is present between the n-type semiconductor layer 122 and the p-type semiconductor layer 124, an electric field can be applied to the optical modulation layer 123 efficiently, and thereby the QCSE can be used efficiently. In the semiconductor optical modulator 120, for example, the impurity concentration of each of the n-type semiconductor layer 122 and the p-type semiconductor layer 124 is 1017 cm−3 or more and the impurity concentration of the undoped layer (i layer) is 1016 cm−3 or lower.
A semiconductor MZM using the semiconductor optical modulator having the p-i-n-type layer structure has a problem that the optical transmission loss in the p-type semiconductor layer is large as compared to the case of using the LN modulator. The optical transmission loss is a problem that cannot be ignored because there is a case of using the semiconductor MZM having a length of a millimeter order in the traveling direction of the signal light.
Hence, a semiconductor optical modulator having the n-i-n-type layer structure, which does not contain a p-type semiconductor layer and a semiconductor optical modulator having the n-SI-i-n-type layer structure, which does not contain a p-type semiconductor layer have been proposed (for example, see Patent Document 2). Here, the “SI layer” refers to a semi-insulating semiconductor layer (a semi-insulating layer), and the impurity concentration of the SI layer is negligibly small. These structures make it possible to reduce the optical transmission loss. However, in these semiconductor optical modulators, an electric field applied to the undoped layer (i layer) is small as compared to the semiconductor optical modulator having the p-i-n-type layer structure.
Hence, a semiconductor optical modulator as shown in FIGS. 13A and B has been proposed (for example, see Patent Document 3). FIG. 13A is a perspective view of the semiconductor optical modulator, and FIG. 13B is a cross-sectional view taken along the line I-I of FIG. 13A. In FIGS. 13A and B, identical parts are indicated with identical numerals and symbols. As shown in FIGS. 13A and B, in a semiconductor optical modulator 130, formed is a Highmesa waveguide structure in which a first n-InP cladding layer 132, an optical modulation layer (i layer) 133, an SI cladding layer 137, and a second n-InP cladding layer 135 are laminated on a substrate 131 in this order. In the semiconductor optical modulator 130, a part of the second n-InP cladding layer 135, which is an interval having a certain length in the traveling direction of the signal light, serves as a p-type semiconductor region 138. A passivation film 139 is arranged at the side surfaces of the Highmesa waveguide structure. The arrow C indicates the boundary between the Highmesa waveguide structure and the passivation film 139. In FIG. 13A, illustration of the passivation film 139 is omitted.
FIG. 14B shows an example of the band diagram of the semiconductor optical modulator 130. In a band diagram 130a, band diagrams 135a, 138a, 137a, 133a, and 132a are band diagrams respectively corresponding to the second n-InP cladding layer 135, the p-type semiconductor region 138, the SI cladding layer 137, the optical modulation layer (i layer) 133, and the first n-InP cladding layer 132 of the semiconductor optical modulator 130 shown in FIG. 13. In the semiconductor optical modulator 130, by suitably setting the thickness of the p-type semiconductor region 138, the optical transmission loss can be reduced, and an electric field can be applied to the optical modulation layer (i layer) 133 sufficiently.